(b)
We know that only even prime is 2, then
$(2)^{2} - \lambda(2) + 12 = 0\ \Rightarrow \ \lambda = 8$ …(i)
and $x^{2} + \lambda x + \mu = 0$ has equal roots.
$\therefore\ \lambda^{2} - 4\mu = 0$ or $(8)^{2} - 4\mu = 0$ [ from Eq. (i)]
$\therefore\ \mu = 16$